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WEDNESDAY, Oct. 21, 2020 (HealthDay News) — Interval breast cancers (IBCs) account for one-fourth of breast cancers detected in routinely screened women, according to a study published online Sept. 25 in JAMA Network Open.

Saroj Niraula, M.D., from the University of Manitoba in Winnipeg, Canada, and colleagues compared tumor characteristics of breast cancers diagnosed within two years of a normal screening mammogram and those of screen-detected breast cancers (SBC). Additionally, breast cancer-specific mortality was compared between IBC and SBC among women diagnosed between January 2004 and June 2010.

The researchers found that among 69,025 women (aged 50 to 64 years), there were 212,579 screening mammograms during the study period, but there were 1,687 breast cancer diagnoses (705 SBC, 206 IBC, 275 noncompliant, and 501 detected outside the screening program). Additionally, there were 225 deaths, including 170 breast cancer-specific deaths. Compared with SBC, IBC were more likely to be of high grade and estrogen receptor-negative (odds ratios, 6.33 [95 percent confidence interval, 3.73 to 10.75; P < 0.001] and 2.88 [95 percent confidence interval, 2.01 to 4.13; P < 0.001], respectively). Breast cancer-specific mortality was higher for IBC during a median follow-up of seven years compared with SBC (hazard ratio, 3.55; 95 percent confidence interval, 2.01 to 6.28; P < 0.001). The two groups had similar non-breast cancer mortality (hazard ratio, 1.33; 95 percent confidence interval, 0.43 to 4.15).

“Substantial compromise in outcomes of IBC compared with SBC reflects the differences in natural history of the two types of cancers and highlights inadequacies in current breast cancer screening practice,” the authors write.

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